1. Field of the Disclosure
The present invention relates to a motor, and more particularly, to relates an outer type motor. In particular, the present invention relates to a motor with an increased efficiency and an improved cogging torque performance by minimizing leaked magnetic flux. Further, the present invention relates to a motor usable for a direct drive type washing machine and a washing machine using the same.
2. Discussion of the Related Art
Generally, a motor transfers a rotating force of a rotor to a rotating axis such that the rotating axis drives a load. For example, the rotating axis is connected to a drum of a washing machine to drive the drum and is connected to a fan of a refrigerator to drive the fan such that a cooling air is supplied to a necessary space.
On the other hand, in such a motor, the rotor rotates by an electromagnetic interaction with a stator. To this end, a coil is wound on the stator and a current is applied the coil, such that a rotating magnetic field is generated between the coil wound on the stator and a permanent magnet of the rotor, thereby rotating the rotor.
Hereinafter, a structure of the rotor and the stator of the motor will be described with reference to the accompanying drawings.
FIG. 1 is a cut perspective view showing a portion of a motor according to the related art. FIG. 2 is a graph schematically showing a magnetic flux density of a motor according to the related art.
As shown in FIG. 1, a stator 10 of a motor is configured to include a stator core 12 that includes a coil 14 and a plurality of teeth 16 on which the coli 14 is wound and an insulator (not shown) that insulates between the coil 14 and a stator core 12.
The rotor 20 is provided to surround the stator 10 and the rotor 20 includes a side wall part 24 and a base part 22. A central part of the base part 22 is connected with a rotating axis (not shown) to be rotated and an inner side of the side wall part 24 is magnetized with a permanent magnet 28 provided in a circumferential direction. A plurality of permanent magnets 28 are provided along the circumferential direction of the side wall part 24. On the other hand, the side wall part 24 is provided with a bending part 26 that supports and magnetizes the lower portion of the permanent magnetic 28.
As described above, in the motor according to the related art, the magnetic flux from the permanent magnet 28 of the rotor 20 does not enter the stator 10 but enters an adjacent rotor 20, thereby generating a leaked magnetic flux. In other words, there is a problem in that the efficiency of the motor is degraded due to leaked magnetic flux.
More specifically, the side wall part 24 and the base part 22 of the rotor 20 are formed of a magnetic material. Thereby, there is a problem in that the magnetic flux due to the permanent magnet 28 is leaked through the side wall part 24 or the base part 22, not the stator 10.
In particular, when the bending part 26 for fixing the permanent magnet 28 is formed, there may cause a problem in that the magnetic flux is directly leaked through the bending part 26.
Further, the problem of the leaked magnetic flux more increases as a height of the permanent magnet 28 is larger than a stacked height of the stator 10 facing the permanent magnet 28, as shown.
In other words, when a lower end portion of the permanent magnet 28 more approaches the rotor 20, in particular, the base part 22 or the bending part 26 than the stator 10, there is a problem in that the leaked magnetic flux more increases.
Further, the bending part 26 is formed by a press processing. Since this is not a machine processing, the dimension is not very precise. Therefore, when a permanent magnet 229 is magnetized to the bending part 26 molded by the press, a deviation in a fixed height of the permanent magnet 28 may be caused.
In the related art, the permanent magnet 28 is formed in a plurality of pieces and is coupled with to the inner side wall of the rotor 20 in a circumferential direction. The permanent magnet 28 is magnetized so that an N pole and an S pole are alternately formed along a circumferential direction.
And, an inner side surface of the permanent magnet 28 is formed in a curved form and an interval (gap) with the stator 10 is constantly formed along the circumferential direction. The permanent magnet in such a form is generally referred to as a C type permanent magnet.
Therefore, as shown in FIG. 2, the magnetizing waveform of the permanent magnet 28 appears as an approximately square wave form according to an angle and as a trapezoidal form, such that a point where magnetic pole suddenly changes along the circumferential direction of the rotor 20 is formed. In other words, the magnetic pole suddenly changes at a point passing from a specific permanent magnet 28 to a neighboring permanent magnet 28.
In general, in a brushless DC (BLDC) motor, the rotating speed of the rotor 20 is controlled by an inverter. In other words, after an alternating current voltage, which is a commercial voltage, is converted into a direct current voltage, the direct current voltage is converted into a three phase (u, v, w) alternating current voltage again and then, the three phase alternating current voltage is applied to the motor.
And, the magnitude and frequency in voltage applied to the motor are controlled by controlling a magnitude of a duty ratio as a pulse width modulation waveform.
Meanwhile, a scheme of driving the inverter may be divided into a square wave driving scheme and a sine wave driving scheme. This is divided according to whether a conduction angle is 180° or 120° in 6 switching sequences of the three phase inverter.
Herein, when the inverter has the conduction angel of 120°, since harmonic component is small, it more approaches the sine wave, such that it mainly uses the sine wave driving scheme so as to drive the motor for the washing machine.
However, in the motor according to the sine wave driving scheme and the motor having a magnetizing form, the cogging torque, etc., occurs due to the square wave magnetizing form of the motor. A torque ripple pulsating other than a torque driving the motor occurs due to the problem, which results from problems of reducing the efficiency of the motor and increasing the vibration and noise.
In other words, the cogging torque occurs due to the sudden change of the interaction of the slot and the permanent magnet 28 between the teeth on which the coil 14 is wound while the motor rotates and as the magnetizing form of the permanent magnet 28 approaches the square wave, there is a problem in that the cogging torque more increases.
Therefore, in a general direct drive type motor, a need exists for a scheme to solve the problems of the cogging torque increased due to the square wave magnetizing form of the permanent magnet and thus, the vibration and noise.